1. Field of the Invention
The invention relates to photolithography technology, and in particular to an optical proximity correction (OPC) photomask for fabrication of contact holes.
2. Description of the Related Art
In the fabrication of semiconductor devices, patterns corresponding to various features (e.g., contact holes) are typically transferred from a photomask used in the photolithography process onto a photoresist on a silicon wafer. Thereafter, the feature patterns are transferred to the silicon wafer by an etching process. However, due to diffraction or other causes, the feature patterns on the photomask cannot successfully be imprinted onto the photoresist, resulting in increasing difficulty for feature fabrication. Accordingly, one of the goals in semiconductor device fabrication is to precisely reproduce original designed patterns onto the photoresist on the wafer by the photomask.
Currently, several OPC photomask have been developed, such that the original designed patterns can be precisely transferred onto the photoresist. A common OPC photomask, which is widely known, has sub-resolution scattering bars. Since the scattering bar patterns on the OPC photomask are so thin, they cannot be transferred onto the photoresist after performing an exposure process, but can enhance pattern sharpness during the lithography process. For example, U.S. Pat. No. 6,238,825, discloses a photomask with alternating arrangements of scattering bars to enhance the resolution of photolithography by OPC.
However, the pattern size and pitch corresponding to the feature patterns are narrowing with the increase of integration of integrated circuits (ICs). As a result, the scattering bar patterns are becoming very close to the feature patterns (e.g., contact hole patterns) on the photomask, and thus the process window of the photomask is reduced and the adjacent imprinted patterns are easily linked together, such that the feature patterns cannot be successfully imprinted onto the photoresist.
Accordingly, there exists a need in the art for development of a novel OPC photomask, which is capable of eliminating the deficiencies mentioned above.